SU1201686A1 - Capacitance level meter - Google Patents

Abstract

A CAPACITIVE LEVEL METER containing a two-electrode sensor connected to a high-frequency generator, a capacitor-to-voltage converter connected to the output of the sensor and made in the form of an operational amplifier with a negative feedback link, and a detector connected to the output of the converter. characterized in that, in order to improve accuracy with di. A station device is inserted into it, to which a single input of the op amp is connected to a common point (body), to another source of the reference voltage, its output is connected to the non-inverting input of the amplifier via a detector, and the negative feedback link is delta-connected capacitors and a resistor, the first vertex of which is formed by a resistor and a capacitor, is connected to the output of an operating unit (L amplifier, second, formed by a resistor and another capacitor is connected to its inverting input, and a third vertex formed by two capacitors is connected to a series-connected sensor and an output of the generator.

Description

The invention relates to instrumentation, namely level control devices, and may find application in various industries and power engineering.

The purpose of the invention is to increase accuracy in remote measurement.

FIG. 1 shows a structural diagram of the meter; in fig. 2 is equivalent to a capacitance-to-voltage converter circuit; in fig. 3 waveform at the output of the converter. H

Capacitive level meter includes (fig. IT capacitive two-electrode sensor 1, the interelectrode capacitance of which Cd is determined by the position of the level, high-frequency generator 2, capacitor-to-voltage converter 3, made on the basis of operational amplifier 3 with negative feedback link Z, threshold device A and detector 5, the output of which is U (n ,, x is simultaneously the output of the meter.

The feedback link 3 of amplifier C is made in the form of a triangle formed by two capacitors C and Cp and a resistor R, with one peak (R, C) connected to the output of the amplifier, the second (R, Cp) with its inverting input, and between the third peak (C, Cp) and the common point of the measuring circuit (case) are connected in series with the sensor and the generator output, they are connected to the point (C, Cp) by a shielded cable, the screen of which is connected to the common point of the circuit (case).

Capacitive sensor 1 includes measuring electrodes 1 and 1 that are fixed in parallel, and one of the electrodes can be grounded, for example, when monitoring an electrically conductive liquid, when the liquid itself acts as one of the electrodes

In general, the sensor capacity is equal to

(one)

СД САО + ЛС

-BUT

where Sdo is the capacity of the sensor in the absence of liquid;

if the linear parameters of the sensor are constant along the entire height;

coefficient of proportionality, taking into account the size, performance and distance between the electrodes. In order to provide a serial connection, the output of the generator 2 with sensors when one of the sensor electrodes is grounded, a transformer 1 is inserted in it, the primary winding of which is connected between the grounded electrode and the generator output,

and secondary, between the second electrode and the common point of the capacitors C and Cf of the feedback link.

FIG. 2a shows the equivalent circuit of the converter 3 capacitance

in the voltage indicating the inclusion of destabilizing capacitances of the communication line C and, moreover, one of them is connected between the case and the common point (c, Cf) and the other capacity C

Shunts generator output 2.

Operational amplifier 3 of the converter employs two functions: amplification of alternating current signals, constant voltage amplification.

As a DC amplifier, it is a repeater (Fig. 2c), in which a constant voltage is applied to a non-inverting input, and the inverting input is connected to a negative feedback circuit resistor R. With a large gain at its input and output, the voltage is the same.

As an AC amplifier, it is an inverting amplifier, the gain of which is determined by the capacitance value of sensor C, and the parameters of the feedback link. The amplitude value of the AC voltage at the output of the converter 3 is

. Ca

one

(2)

U Ur. "

  ,

RCCpuJ where C Cp + C + Cd + C; u) 2Tf;

f is the generator voltage frequency — the generator voltage amplitude.

. The resulting voltage at the output of converter 3 includes the constant and variable components and is equal to

, and,., § ---. C--.

(3)

VDV;

RiJCC When R and Cp are increased, RoCCp and expression (3) takes on the form CA. This condition is not difficult to fulfill, since the capacitor Cp is separable and the capacitance can be chosen sufficiently large, substantially larger than the capacitance of cable C. When this condition is fulfilled, the output voltage of the converter is determined by the ratio Cd / C and does not depend on the capacitance of the cable Cc. Capacity Sk shunts the output of the generator and, when the generator is executed as an EMF source, does not affect the measurement accuracy. The output signal of the converter Uj is fed to the input of the threshold device 4, to another input of which the reference voltage Uon is applied, whose magnitude is chosen equal to Cdr. At the output of the threshold device, voltage pulses appear (Fig. 3), which are detected by the detector 5. The voltage at the detector output is U, v (Uj-Uow) K5, where K5 is the detector transmission coefficient. The voltage from the detector output is fed as a feedback signal to the non-inverting input of amplifier 3. With a large K5 gain, the maximum value of the output signal from the converter, Up, §-, tends to the value of and "n. Taking into account expression (5), the output voltage of the detector is dSd k. C I + KS Consequently, the output signal of the detector is proportional to the change in the capacitance of the sensor, which depends on the level, does not depend on the capacity of the communication line when the specified conditions are met, the detector characteristics are significantly reduced, while the non-linearity of the detector does not affect the measurement accuracy at all.

Claims (1)

A CAPACITIVE LEVEL METER containing a two-electrode sensor connected to a high-frequency generator, a capacitance to voltage converter connected to the sensor output and made in the form of an operational amplifier with a negative feedback link, and a detector connected to the converter output, characterized in that, in order to increase accuracy during remote measurement, a threshold device is introduced into it, the output of the operational amplifier is connected to one input of which relative to a common point (case), to another source to the reference voltage, its output through the detector is connected to the non-inverting input of the amplifier, and the negative feedback link is made in the form of two capacitors and a resistor connected by a triangle, the first vertex of which is formed by a resistor and a capacitor connected to the output of the operational amplifier, the second is formed by a resistor and another a capacitor connected to its inverting input, and a third peak formed by two capacitors connected to a series-connected sensor and the output of the generator. Sec. SU 1201686